Technically speaking, Win XP will provide slightly better performance since the operating system itself uses less resources than Win 7. It would be hard to tell the difference though without resorting to benchmarks.
Performance will basically boil down to which version of DirectX you will be playing with. DX9 is less demanding than DX10 or DX11 so playing games in DX9 mode will give you the best performance. DX10 offers very little improvements over DX9, but it has a somewhat large decrease in performance. DX11 is more demanding than DX10, but the improvement in eye candy vs. DX9 is greater than DX10 vs. DX9.
Win XP is limited to DX9; you cannot install DX10 or DX11. That means you will not be able to play Just Cause 2 and Battlefield 3 because they have dropped support for DX9 and they represent the 1st nail in DX9's coffin.
Win 7 allows you to choose which DX mode to play the game in, assuming the game supports that particular DX mode. Meaning you cannot play Just Cause 2 in DX9 mode because the game doesn't support DX9. You cannot play Fallout: New Vegas in DX11 mode, cause the game does not support DX11.
Vista... I've read several posts where people had issues playing games that worked perfectly fine with Win XP, but had issues with Vista. However, when they switched to Win 7 those games were working properly.
The Death of DX9 will be long and drawn out; unlike the Death of DX8 which was relatively quick. A lot of people still use Win XP (like me) and a lot of people still use cards that only supports DX9. More importantly games designed for both PC and consoles (like the Xbox 360) will probably just have DX 9 support for the PC version.
Crysis 2 is an example of a current DX9 only game. It does not support DX10 or DX11. It was designed to use DX9 'cause the Xbox 360 only uses DX9.
I've tried all three...XP, Vista and 7 and I can safely say that Win 7 was the best bet for me. Plus, with Win 7 you get the option of using all the DX versions (9, 10 and 11) which XP will not offer as it only supports DX9.
Using Vista was pretty buggy for me while gaming coz it used up pretty much all the resources and still wanted more! Win 7 has been the best of the lot and I'd call it the Win XP of today!
there should be no difference between xp sp3 and win 7 home premium 32 bit versions of both.
win7 64 bit will use slightly more ram and cpu resources but is more efficient when doing photoshop and other math intensive apps.